1. Field of the Invention
The invention relates to ablative insulation, particularly insulation to protect the interior of a rocket motor from the combustion products of burning propellant. More particularly, the present invention relates to thermoplastic elastomeric ablative insulation which retains good mechanical properties at low temperature.
2. Technology Review
The combustion of a propellant in a rocket motor creates a hostile environment characterized by extremely high temperature, pressure, and turbulence. The combustion temperature within the motor often exceeds 6,000.degree. F., and the pressure within the motor frequently exceeds 1,000 psi. Gas velocities typically range from Mach 0.2 in the inlet region to Mach 10+ at the aft end of the rocket motor nozzle. This environment is particularly hostile in a solid rocket motor because its combustion gas contains chemical species and particulates which tend to physically and chemically erode exposed rocket motor nozzle components. While the combustion of a rocket propellant is usually brief, the conditions described above can destroy insufficiently protected or inferior rocket motor parts prematurely and jeopardize the mission of the motor.
Parts of a rocket which are exposed to the high temperatures, pressures, and erosive flow conditions generated by the burning propellant must be protected by a layer of insulation. Various materials have been tried as insulation, such as silica dioxide, glass, or carbon fiber reinforced silicone and/or polyisoprene elastomers, but reinforced resin composite materials are most commonly used. These include phenolic resins, epoxy resins, high temperature melamine-formaldehyde coatings, ceramics, polyester resins and the like. These materials, when cured, usually become rigid structures which crack or blister when exposed to the rapid temperature and pressure changes occurring when the propellant is burned.
The best rocket insulation materials previously known to the art are elastomeric polymers reinforced with asbestos, polybenzimidazole fiber, or polyaramid fiber. These compositions are ablative insulation because they are partially consumed during combustion, but nevertheless they provide protection for the rocket motor. Such materials are capable of enduring in a rocket motor long enough to allow complete combustion of the propellant. Asbestos-reinforced elastomeric insulation is the subject of U.S. Pat. No. 3,421,970, to Daley et al., issued Jan. 14, 1969, and U.S. Pat. No. 3,347,047, to Hartz et al., issued Oct. 17, 1967.
Environmental and health concerns have led manufacturers to seek an acceptable replacement for the asbestos in rocket motor case insulation. One alternative elastomeric insulation contains aramid polymer fibers in combination with a powder filler. That insulation is disclosed in U.S. Pat. No. 4,492,779, assigned to Morton Thiokol, Inc., now known as Thiokol Corporation. A third alternative is elastomeric insulation which contains polybenzimidazole polymer fibers in combination with a powder filler. That insulation is disclosed in U.S. Pat. No. 4,600,372, also assigned to Morton Thiokol, Inc. (See also U.S. Pat. No. 4,507,1665.)
Another problem with existing rocket motor insulation is the expense and difficulty of fabricating an insulator and installing it, either as one piece or in sections, within a rocket motor casing. The problems of fabricating thermosetting resinous insulation which is not capable of being cast are described in U.S. Pat. No. 3,177,175, issued to Barry, Jr., on Apr. 6, 1965. While uncured thermosettable resins and elastomers can be formed under heat and pressure in a matched metal die mold, they can only be formed before they cure to a thermoset condition. Typically, both heat and pressure must be exerted during the curing reaction to fuse overlapped segments of insulation into a smooth-surfaced, integral layer. For larger solid rocket motors, precured elastomeric material is often used as insulation. This cured material is laid up and joined within a rocket motor casing with an adhesive to fabricate an insulation member. It is then necessary to machine the insulation to provide a smooth surface which does not have overlapped sections. A further disadvantage of using curable resinous or elastomeric insulation is the time required to cure the insulation sufficiently--between several hours and several days.
To alleviate some of the problems of handling thermosetting materials, insulation consisting of filled polyolefins such as polyethylene or polypropylene has been proposed. Besides the obvious fabrication economies of working with thermoplastic insulation, the prior art has recognized the theoretical superiority of thermoplastic resins for ablative insulation because they undergo endothermic pyrolysis, carrying heat away from the insulation. Thermoplastic resins also have high specific heats, and their pyrolysis products have high specific heats and low molecular weights. The theoretical superiority of thermoplastic resins is recognized in U.S. Pat. No. 3,395,035, issued to Strauss on Jul. 30, 1968 (column 6, lines 39-53); and U.S. Pat. No. 3,397,168, issued to Kramer et al., on Aug. 13, 1968 (column 2, lines 15-19; column 3, lines 4-5).
Thermoplastic resin-based material readily melts and flows when subjected to heat. (See the Kramer et al. patent previously cited, column 1, line 64 to column 2, line 4.) Therefore, the art teaches that thermoplastic resins used in ablative insulation must be combined with thermosetting resins and impregnated into a refractory or fiber matrix to prevent the insulation from melting and running off when exposed to the extreme heat and erosion of a rocket motor.
The previously cited Strauss patent, particularly at column 2, lines 55-60, described resin-impregnated open-celled porous ceramic material as such a matrix. While Strauss contemplates the use of the thermoplastic materials as the impregnant, most of the resins actually listed are thermosetting, and the reference indicates at column 6, lines 28-38 that thermosetting resins are preferred. Kramer et al. says that thermoplastic resins impregnated in the ceramic matrix cause the ceramic to crack under thermal shock. Column 6, lines 63-66, of Kramer et al. suggests that the cracking problem can be alleviated by impregnating the ceramic matrix with a thermosetting resin before impregnating it with a thermoplastic resin. This minimizes the amount of thermoplastic resin actually present in the insulation, and does not avoid the fabrication problems of thermosetting insulation because the ceramic matrix itself is not thermoplastic. It must be molded (and cured) or machined into the necessary configuration. The working examples of Kramer, et al. do not use any thermoplastic resin.
Another approach is found in U.S. Pat. No. 3,314,915 issued to Baughman, et al., on Apr. 18, 1967. Here, a composite of asbestos and nylon fiber is impregnated with a thermoplastic binder, after which the material is further impregnated with a much larger amount of thermosetting resin (see column 2, lines 45-68). Thus, the insulation contains predominantly thermosetting resin as opposed to thermoplastic resin.
The Kramer et al. patent previously discussed briefly refers to a thermoplastic resin composition not protected by a refractory matrix. In column 3, lines 39 to the end, Kramer demonstrates that polyethylene filled with silica can be melt cast, but cannot even briefly withstand an assault with an oxyacetylene torch.
Several references disclose insulation which is thermoplastic before being cured, but which is cured to form a thermosetting product. (Such materials are distinguished herein from true thermoplastic materials which can be subjected to heat and pressure without curing and thus losing their thermoplastic properties.) For example, in U.S. Pat. No. 3,472,812 issued to Byrne, et al. on Oct. 14, 1969, thermoplastic polymers are suggested, but the insulation appears to be cross-linked before being installed. U.S. Pat. No. 3,459,701 issued to Chandler, et al. on Aug. 5, 1969, discloses copolymerization of a polyamide resin and epoxy resin to form a reinforced composite for protecting rocket motor launch structures from high temperature and gas flow rates. The reference suggests at column 4, lines 69-75, that the two parts of the composition be packed separately and mixed just before use, suggesting a thermosetting material which cures when the two portions are mixed. U.S. Pat. No. 3,562,304, issued to Tucker, Feb. 9, 1971, indicates that a synthetic rubber based insulation can be extruded or formed as sheets (which is true of rubber compositions generally, even though they are thermosetting after being cured under heat and pressure). See also U.S. Pat. No. 4,596,619, issued to Marks on Jun. 24, 1986. This patent teaches that heat and pressure must be applied to the insulation after it is laid up to cure it and fuse the overlapped edges of insulation into an integral, smooth-surfaced member. Also, the tacky ribbon of material must be carefully handled to avoid contamination with dust, can only remain tacky for a relatively short time, and cannot be returned to a tacky state once it begins to cure. The material must be formed, extruded, and applied very promptly to successfully use this technique.
A serious concern for rocket motors, such as tactical motors, which must be able to operate in extremely cold temperatures, as low as -65.degree. F., is the possession of good strain capability. Insulation, whether thermosetting or thermoplastic, often becomes brittle and cracks at extremely cold temperatures. Under such conditions, the rocket motor may fail and break apart.
It would be a significant advancement in the art to provide thermoplastic elastomeric ablative insulation materials having good processing characteristics, which also possess good mechanical characteristics at extremely cold temperatures.
Such thermoplastic elastomeric ablative insulation materials for low temperature application are disclosed and claimed herein.